This invention relates generally to optical encoders using a movable scale with optical slits to interrupt light shining on a photodetector. The electrical signals generated by the photodetector indicate velocity, direction of movement and position of the scale. More particularly, this invention relates to improvements which provide a simple and positive adjustment of the phase angle between the electrical signals generated by the optical encoder.
Optical encoders producing electrical signals corresponding to rotary or axial movement of mechanical parts are known in the art. These devices have been used as tachometers and position sensors for servomechanisms.
Generally, optical encoders have consisted of a stationary housing having a light source in alignment with a phototransducer array. A code disk or scale containing alternating opaque and translucent regions was mounted on a rotating shaft or moving part in such a manner as to interrupt the light beam between the source and the array. Movement of the scale caused the alternating opaque and translucent regions on the disk or scale to block and pass light to the phototransducer array. The pulses of light caused the phototransducer array to produce generally triangular waveform electrical signals having a frequency related to the velocity of the scale. Scale position could be determined by electrically counting the number of cycles effectively yielding a distance of position indication.
To increase position measurement resolution, the alternating opaque and translucent regions had a thin rectangular shape and were positioned so that the shortest dimension of this shape was parallel to the locus of scale movement. In this manner, the maximum number of pulses per unit scale movement was obtained.
A relatively large translucent region was required to provide a usable electrical power output from the phototransducer array. However, increasing the area of the translucent region decreased position detection resolution. To provide sufficient power output and sufficient resolution, a mask containing a plurality of alternating opaque and translucent regions was placed over the phototransducer array. The dimensions of the opaque and translucent regions in the mask matched those of the scale. When adjacent translucent regions of the moving scale aligned with those on the fixed mask, light would shine through a plurality of concurrently aligned translucent regions on to the phototransducer array, thus providing nearly full electrical power output. As the scale moved so that its opaque regions coincided with the translucent regions of the mask, all light to the phototransducer array would be blocked. In this manner, the number of pulses per unit movement of scale, and thus overall resolution, was determined by the width of the opaque and translucent regions measured along the locus of scale movement. Consequently, fine divisions of position could be attained without reducing power output.
To determine direction of scale movement, two photodetectors were incorporated in the phototransducer array. A fixed mask containing two sets of alternating opaque and translucent regions was placed over the phototransducer array. The pitch or distance between corresponding points on successive opaque regions was identical for both mask sets and the scale. However, the mask sets were spaced so that alignment of the translucent regions of the scale with those of the one set was not coincident with alignment with the second set. Generally, the mask sets were offset a distance equal to one quarter of the pitch which made the pulses from one photodetector lag the pulses from the other by 90 electrical degrees or one quarter of a complete cycle. This 90 electrical degree relationship is commonly called quadrature. In this manner, movement of the scale caused phased signals from the photodetectors. By electrically determining which signal lagged the other, direction of the scale movement was identified.
A major drawback with the type of encoder described was the criticality of alignment between the alternating opaque and translucent regions of the scale and those on the mask. Alignment had to be maintained in three dimensions to provide proper operation including maximum power output, regularity of waveform and repeatability of phase relationship.
One possible source of error was lack of a parallel relationship between the face of the scale and the face of the mask. In one prior patent, U.S. Pat. No. 4,224,514 to Weber, et al. describing an optical shaft encoder, errors so produced were minimized by forming the scale of a thin flexible stainless steel disk which was seated in a special guide formed by a surrounding rim and two thin mylar sheets disposed on either side of the stainless steel disk. The guide maintained the scale parallel to the mask even in the event of a slight misalignment of the shaft to which the scale was attached.
Another possible source of error was lack of a parallel relationship between the alternating opaque and translucent regions on the scale and those on the mask. In rotary applications when the scale takes the form of a disk mounted to a rotating shaft, this misalignment is an eccentricity error.
The Weber, et al, encoder attempted to minimize eccentricity error by using four photodetectors arranged in a rectangular pattern behind the mask. The electrical signals from the two photodetectors at diagonally opposite corners of the rectangle were differentially combined to produce one phase signal. The electrical signals from the remaining two photodetectors were similarly combined to produce the second phase signal. The differential combination scheme was said to nullify eccentricity errors. The Weber, et al. encoder had the disadvantage of requiring a relatively complex electro-mechanical system to overcome possible signal phase errors.
In U.S. Pat. No. 4,266,125 to Epstein, et al., an optical encoder was disclosed which attempted to nullify eccentricity errors by using three separate light beams. Three light sources and collimating lenses were used to direct the beams through the scale and the mask. A detector assembly having siamesed and truncated lenses was used to focus the beams on the photodetectors. The Epstein encoder had the disadvantage of requiring a complex light source arrangement and special lenses to overcome eccentricity errors.
Neither of these systems provided a simple adjustment of the encoder to eliminate alignment errors or adjust phase angle. Without such adjustment, error introduced by manufacturing tolerances could not be eliminated. Decreasing the dimensions of the opaque and translucent regions on the scale and mask to obtain higher resolution had a practical limit which was reached when tolerances affecting relative position of the scale and mask exceeded the dimensions of the translucent regions.
In the common assignee's co-pending application Ser. No. 555,591 now U.S. Pat. No. Optical Encoder Apparatus And Methods, an optical encoder was disclosed which provided a simple alignment adjustment. In that device, a housing held on a single axis a light source, a collimating lens, a mask and a phototransducer array. The housing contained a horizontal U-shaped notch through which the edge of the scale passed such that the alternating opaque and translucent regions of the scale interrupted the light beam. The housing also contained a cylindrical bore for rotatably mounting the encoder on a mounting post perpendicular to scale movement. On the mask were two sets of microlines which formed alternating opaque and translucent regions matching those on the scale. These sets were positioned side by side on the mask with the microlines of each perpendicular to the locus of scale movement. The width of the opaque region separating the two sets determined offset and therefore phase angle. The mask was positioned on the housing so that the center of this separating opaque region coincided with the center of housing rotation. Alignment was adjusted by rotating the housing on the mounting post until the rectangular shaped alternating opaque and translucent regions on the mask were parallel to those on the scale. After adjustment, the housing was securely clamped to the mounting post.
This encoder provided a simple method of eliminating alignment or eccentricity errors. By maximizing the instantaneous light transmission area when translucent regions are aligned, maximum power output was attained. The adjustment also had secondary effects on electrical waveform shape and phase angle. However, rotation of the housing in either direction provided unilateral adjustment which could only decrease phase angle. Thus, none of the encoders described above had a positive and simple adjustment for phase angle.
Phase angle adjustment was addressed in U.S. Pat. No. 3,460,275 to D. H. Trump. In that device a parabolic reflector behind a single light source was used to shine two separate light beams on two photodetectors. Axial movement of the reflector relative the light source changed the parallel, convergent or divergent relationship between the beams. This adjustment changed the effective distance between the mask sets and therefore phase angle. The device had the disadvantage of requiring a special and expensive parabolic reflector which was mounted inside the optical encoder housing. Adjustment required disassembly of the encoder. Also, a friction fit of the reflector within a housing bore was relied upon to maintain the adjustment.
A hitherto unsolved need in prior devices was for a simple and sure adjustment which maintained a repeatable and reliable phase angle setting and provided quick and accurate calibration during final assembly of the host equipment using the encoder.